Allow rewriting urls for network images

This is useful to apply a base url, but also could be used to swap domains or force an https schema

Author: erickok

example/lib/main.dart

@@ -178,7 +178,8 @@ class _MyHomePageState extends State<MyHomePage> {
               altWidget: (alt) => Text(alt),
             ),
             (attr, _) => attr["src"] != null && attr["src"].startsWith("/wiki"):
-                networkImageRender(baseUrl: "https://upload.wikimedia.org"),
+                networkImageRender(
+                    mapUrl: (url) => "https://upload.wikimedia.org" + url),
           },
           onLinkTap: (url) {
             print("Opening $url...");

lib/image_render.dart

@@ -90,14 +90,13 @@ ImageRender assetImageRender({
 
 ImageRender networkImageRender({
   Map<String, String> headers,
-  String baseUrl,
+  String Function(String) mapUrl,
   double width,
   double height,
   Widget Function(String) altWidget,
 }) =>
     (context, attributes, element) {
-      final src =
-          baseUrl != null ? baseUrl + _src(attributes) : _src(attributes);
+      final src = mapUrl?.call(_src(attributes)) ?? _src(attributes);
       precacheImage(
         NetworkImage(
           src,